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Pythium and Phytophthora root rots of soybean

Editor’s note: This article is from the archives of the MSUCrop Advisory Team
Alerts. Check the label of any pesticide referenced to ensure your use is
included.

Despite
recent dry weather, growers may still be seeing evidence of root rot
diseases in soybean fields, especially those that were very wet earlier
in the year. Pythium root rot and Phytophthora root rot can develop in
poorly-drained or saturated soils or soil that suffers from compaction.
No-till areas with fine-textured soils tend to be favorable environments
for these diseases, as the soil can become more easily saturated than
coarser textured soils. The swimming spores, (called zoospores) need
saturated soil to move to soybean roots to cause infection. Seedlings
infected die rapidly, leaving gaps in rows. Soil temperatures in the
range of 50 to 59°F are favorable for Pythium; Phytophthora needs warmer
soil temperatures (around 68°F) for production of zoospores. Heavy
rainfall resulting in flooded fields within a week or so of planting
creates the most favorable conditions for infection. The symptoms are
similar for both diseases . Young plants turn yellow, collapse and die,
leaving gaps in row plantings. In plants that have reached more advanced
growth stages, foliage wilts and turns yellow starting at the bottom of
the plant and moving up. Dead leaves may remain attached. Lower stem
discoloration starts near the soil line and spreads upward. Root tips
and feeder roots are infected, causing root rots. Check plants in low
areas or poorly drained parts of the field for symptoms.

It’s been mentioned several times on our Field Crop CAT Alert
conference calls this year that this is a year when properly tiled
fields will pay off. Improving drainage to remove excess water will
reduce the duration and amount of soil saturation, creating a less
favorable environment for the spores to cause infection. There is no
genetic resistance to Pythium root rot, but there are soybean varieties
with tolerance to Phytophthora sojae that would be worth considering for next year if you have had problems with Phytophthora root rot this season.